Asynchronous pacing is a specialized mode of cardiac pacing where a device delivers electrical impulses to the heart at a fixed, predetermined rate. The term “asynchronous” means the artificial stimulation is not coordinated or synchronized with the patient’s natural heart rhythm. This fixed-rate approach departs from standard pacemaker operation, which typically monitors the heart and only intervenes when necessary. This mode is primarily used for specific, often temporary, clinical situations.
Fixed-Rate Pacing Mechanism
Asynchronous pacing delivers an electrical impulse at a constant interval, defined by a programmed rate (e.g., 70 beats per minute). The device lacks the circuitry to “listen” for native heartbeats and continues to fire regardless of the heart’s intrinsic activity.
This mechanism is reflected in the standard three-letter pacemaker coding nomenclature by the letter “O” in the second position, which denotes the chamber being sensed. For example, VOO stands for ventricular pacing (V), no sensing (O), and no response to sensing (O). AOO paces the atrium without sensing, and DOO paces both the atrium and ventricle asynchronously.
The “O” signifies that the device’s sensing circuit is intentionally inhibited or turned off. This absence of sensing is the defining characteristic that ensures the fixed-rate output, functioning as a simple metronome to maintain a guaranteed minimum heart rate.
How Asynchronous Pacing Differs from Demand Pacing
The fundamental difference between asynchronous and demand pacing lies in the function of the sensing circuit. Demand pacing, the standard mode for most patients, is also known as synchronous pacing because it actively senses the patient’s intrinsic heart rhythm.
In demand modes, such as VVI, the pacemaker senses a natural heartbeat within a set time and is inhibited from firing. The device acts as a backup, only delivering a pace if the heart rate drops below a programmed lower limit. This prevents unnecessary pacing and allows the natural rhythm to take precedence.
Asynchronous pacing operates with its sensing function disabled, ignoring the heart’s intrinsic activity and firing continuously at a fixed rate. This lack of synchronization introduces the risk of “competition,” where the artificial impulse occurs simultaneously with or immediately after a natural beat. This transforms the device from a responsive monitor into a non-stop fixed-rate generator.
Specific Clinical Applications
Asynchronous pacing is typically reserved for temporary use in closely monitored settings where a guaranteed fixed rate outweighs the risks of competition.
Use During Surgery
One common application is during surgical procedures involving electrocautery. Electrocautery generates intense electromagnetic interference that can be “oversensed” by a demand pacemaker. This interference can cause the demand pacemaker to mistakenly inhibit its pacing output. Programming the device to an asynchronous mode (VOO or DOO) ensures the pacemaker ignores this external interference and continues to fire at a safe, fixed rate, preventing a dangerously slow heart rate.
Other Temporary Uses
This mode is also used temporarily when treating severe bradycardia where sensing circuit compromise is a concern, such as immediately following cardiac surgery when temporary pacing wires may be unstable. Additionally, asynchronous pacing can be briefly employed in diagnostic or testing situations to evaluate the effectiveness of the pacing lead itself. By forcing a fixed rate, clinicians confirm the lead successfully delivers the electrical impulse to the heart muscle. This mode is a specialized, time-limited intervention, not a long-term solution.
Monitoring and Safety Considerations
The primary concern with asynchronous pacing stems directly from its lack of sensing, allowing the artificial impulse to fall anywhere within the cardiac cycle. If a pacemaker impulse lands during the T-wave (the repolarization phase of the ventricle), it can trigger the R-on-T phenomenon.
This period is highly vulnerable to electrical stimulation, and an impulse delivered then can induce life-threatening ventricular arrhythmias, such as ventricular fibrillation. Due to this inherent risk of competitive pacing, asynchronous modes are almost never used for permanent, long-term pacing.
This mode is limited to temporary pacing situations in monitored areas like the operating room or the intensive care unit. In these environments, continuous cardiac monitoring allows for immediate intervention if an arrhythmia occurs. The clinical use of asynchronous pacing is a carefully managed trade-off, accepting the risk of competition to guarantee a life-sustaining heart rate when standard demand-pacing modes are compromised.